1. Field of Invention
The present invention relates to a parachute foot support stirrup for preloading the leg muscles of a parachutist prior to his landing on solid ground.
2. Description of Prior Art
Exercising a muscle involves its repeated contraction and relaxation, usually with a view to strengthening it for the time when it may be needed. Strength in this case is the amount of force that the muscle might be able to exert, and elastic devices or springs are usually ideal for the purpose of "exercising".
Preloading a muscle, however, is an entirely different activity requiring similar yet rather distinct devices. When one jumps off a chair, the interval of time from muscle relaxation which begins at the moment of resistance-removal (lift off) to the time of impact is so short that the muscles retain some preload allowing them to absorb the loads of landing impact with respectable if far from perfect efficiency. If one jumped off a higher structure under reduced gravity such as from a building on the moon, then the landing impact would be about the same but the time airborne would greatly increase and the muscles would retain none of of the preload described above. This is exactly what happens in a parachute landing. Impact forces are not all that great and they compare with jumping off from relatively low structures, but the amount of time spent hanging in the harness that supports the torso while it leaves the legs hanging limp is far greater. Come time to absorb landing impact, the limp legs, even if cautiously bent, will normally collapse significantly before becoming able to exert force. Force cannot be exerted against a void and the muscles cannot even begin to load until resistance in the form of terrafirma is encountered. At that moment, they begin to build up force but the process takes time and taxes heavily the time available before the legs become fully collapsed. Some absorption is generated, but it can be greatly increased by using the parachute foot support stirrup of the prresent invention. Landing with straight legs, preloaded or limp, can result in incapacitating injury.
The stirrup of the present invention provides the equivalent of a harness in which the occupant figuratively stands during descent and even in the absence of a will to preload the leg muscles will do just that by virtue of the legs having to support the weight of the body. In the case of a conscious occupant, from a forces-in-action perspective, this arrangement would nearly equal standing on the ground with most body muscles similarly preloaded. Furthermore, because the upper part of the torso is restrained by the harness and/or clothing while the feet are restrained by the stirrup and the two combined form the necessary containment geometry, forces considerably greater than the weight of the body can be exerted at will.
This act would be an expected course to follow just prior to landing when landing can be closely predicted. A more continuous and vaguely aimed effort would apply when the time of landing cannot be closely predicted, such as at night or when landing into forests, etc.
Since exercising devices using elastic materials neither allow the application of larger forces, nor provide containment geometry to prevent the legs from straightening, unless stretched to the limit at which time their use becomes futile, and since the stirrup provides these objectives not only without but specifically excluding the use of elastic materials, the exercising devices made of elastic materials cannot deliver these objectives. The two concepts are totally foreign one to another.